org.apache.openjpa.lib.util.concurrent
Interface Lock

Lock implementations provide more extensive locking
operations than can be obtained using synchronized methods
and statements. They allow more flexible structuring, may have
quite different properties, and may support multiple associated
Condition objects.
A lock is a tool for controlling access to a shared resource by
multiple threads. Commonly, a lock provides exclusive access to a
shared resource: only one thread at a time can acquire the lock and
all access to the shared resource requires that the lock be
acquired first. However, some locks may allow concurrent access to
a shared resource, such as the read lock of a ReadWriteLock.
The use of synchronized methods or statements provides
access to the implicit monitor lock associated with every object, but
forces all lock acquisition and release to occur in a block-structured way:
when multiple locks are acquired they must be released in the opposite
order, and all locks must be released in the same lexical scope in which
they were acquired.
While the scoping mechanism for synchronized methods
and statements makes it much easier to program with monitor locks,
and helps avoid many common programming errors involving locks,
there are occasions where you need to work with locks in a more
flexible way. For example, some algorithms for traversing
concurrently accessed data structures require the use of
"hand-over-hand" or "chain locking": you
acquire the lock of node A, then node B, then release A and acquire
C, then release B and acquire D and so on. Implementations of the
Lock interface enable the use of such techniques by
allowing a lock to be acquired and released in different scopes,
and allowing multiple locks to be acquired and released in any order.
With this increased flexibility comes additional
responsibility. The absence of block-structured locking removes the
automatic release of locks that occurs with synchronized
methods and statements. In most cases, the following idiom should be used:

When locking and unlocking occur in different scopes, care must be
taken to ensure that all code that is executed while the lock is
held is protected by try-finally or try-catch to ensure that the
lock is released when necessary.
Lock implementations provide additional functionality
over the use of synchronized methods and statements by
providing a non-blocking attempt to acquire a lock(tryLock()), an attempt to acquire the lock that can be
interrupted(lockInterruptibly(), and an attempt to acquire
the lock that can timeout(tryLock(long, TimeUnit)).
A Lock class can also provide behavior and semantics
that is quite different from that of the implicit monitor lock,
such as guaranteed ordering, non-reentrant usage, or deadlock
detection. If an implementation provides such specialized semantics
then the implementation must document those semantics.
Note that Lock instances are just normal objects and can
themselves be used as the target in a synchronized statement.
Acquiring the
monitor lock of a Lock instance has no specified relationship
with invoking any of the lock() methods of that instance.
It is recommended that to avoid confusion you never use Lock
instances in this way, except within their own implementation.
Except where noted, passing a null value for any
parameter will result in a NullPointerException being thrown.

A successful lock operation has the same memory
synchronization effects as a successful Lock action.

A successful unlock operation has the same
memory synchronization effects as a successful Unlock action.

Unsuccessful locking and unlocking operations, and reentrant
locking/unlocking operations, do not require any memory
synchronization effects.

Implementation Considerations

The three forms of lock acquisition(interruptible,
non-interruptible, and timed) may differ in their performance
characteristics, ordering guarantees, or other implementation
qualities. Further, the ability to interrupt the ongoing
acquisition of a lock may not be available in a given Lock
class. Consequently, an implementation is not required to define
exactly the same guarantees or semantics for all three forms of
lock acquisition, nor is it required to support interruption of an
ongoing lock acquisition. An implementation is required to clearly
document the semantics and guarantees provided by each of the
locking methods. It must also obey the interruption semantics as
defined in this interface, to the extent that interruption of lock
acquisition is supported: which is either totally, or only on method entry.
As interruption generally implies cancellation, and checks for
interruption are often infrequent, an implementation can favor responding
to an interrupt over normal method return. This is true even if it can be
shown that the interrupt occurred after another action may have unblocked
the thread. An implementation should document this behavior.

lock

void lock()

Acquires the lock. If the lock is not available then
the current thread becomes disabled for thread scheduling
purposes and lies dormant until the lock has been acquired.
Implementation Considerations
A Lock implementation may be able to detect
erroneous use of the lock, such as an invocation that would cause
deadlock, and may throw an(unchecked) exception in such circumstances.
The circumstances and the exception type must be documented by that
Lock implementation.

lockInterruptibly

Acquires the lock unless the current thread is
interrupted.
Acquires the lock if it is available and returns immediately.
If the lock is not available then
the current thread becomes disabled for thread scheduling
purposes and lies dormant until one of two things happens:

The lock is acquired by the current thread; or

Some other thread interrupts the current
thread, and interruption of lock acquisition is supported.

If the current thread:

has its interrupted status set on entry to this method; or

is interrupted while acquiring
the lock, and interruption of lock acquisition is supported,

then InterruptedException is thrown and the current thread's
interrupted status is cleared.
Implementation Considerations
The ability to interrupt a lock acquisition in some
implementations may not be possible, and if possible may be an
expensive operation. The programmer should be aware that this
may be the case. An implementation should document when this is the case.
An implementation can favor responding to an interrupt over
normal method return.
A Lock implementation may be able to detect
erroneous use of the lock, such as an invocation that would
cause deadlock, and may throw an(unchecked) exception in such
circumstances. The circumstances and the exception type must
be documented by that Lock implementation.

Throws:

InterruptedException - if the current thread is interrupted
while acquiring the lock(and interruption of lock acquisition is
supported).

tryLock

boolean tryLock()

Acquires the lock only if it is free at the time of invocation.
Acquires the lock if it is available and returns immediately
with the value true.
If the lock is not available then this method will return
immediately with the value false.
A typical usage idiom for this method would be:

tryLock

Acquires the lock if it is free within the given waiting time and the
current thread has not been interrupted.
If the lock is available this method returns immediately
with the value true. If the lock is not available then
the current thread becomes disabled for thread scheduling
purposes and lies dormant until one of three things happens:

The lock is acquired by the current thread; or

Some other thread interrupts the current
thread, and interruption of lock acquisition is supported; or

The specified waiting time elapses

If the lock is acquired then the value true is returned.
If the current thread:

has its interrupted status set on entry to this method; or

is interrupted while acquiring
the lock, and interruption of lock acquisition is supported,

then InterruptedException is thrown and the current thread's
interrupted status is cleared.
If the specified waiting time elapses then the value false
is returned. If the time is
less than or equal to zero, the method will not wait at all.
Implementation Considerations
The ability to interrupt a lock acquisition in some implementations
may not be possible, and if possible may be an expensive operation.
The programmer should be aware that this may be the case. An
implementation should document when this is the case.
An implementation can favor responding to an interrupt over normal
method return, or reporting a timeout.
A Lock implementation may be able to detect
erroneous use of the lock, such as an invocation that would cause
deadlock, and may throw an(unchecked) exception in such circumstances.
The circumstances and the exception type must be documented by that
Lock implementation.

Parameters:

time - the maximum time to wait for the lock

unit - the time unit of the time argument.

Returns:

true if the lock was acquired and false
if the waiting time elapsed before the lock was acquired.

Throws:

InterruptedException - if the current thread is interrupted
while acquiring the lock(and interruption of lock acquisition is
supported).

unlock

void unlock()

Releases the lock.
Implementation Considerations
A Lock implementation will usually impose
restrictions on which thread can release a lock(typically only the
holder of the lock can release it) and may throw
an(unchecked) exception if the restriction is violated.
Any restrictions and the exception
type must be documented by that Lock implementation.

newCondition

Returns a new Condition instance that is bound to this
Lock instance.
Before waiting on the condition the lock must be held by the
current thread.
A call to Condition.await() will atomically release the lock
before waiting and re-acquire the lock before the wait returns.
Implementation Considerations
The exact operation of the Condition instance depends on the
Lock implementation and must be documented by that
implementation.